1. Field of the Invention
The present invention relates to a series resonant power converter for producing a desired DC voltage for a load, and a method of controlling such series resonant converter.
2. Description of the Prior Arts
In general, a value of a resonant current of a series resonant converter depends on values of an inductance of a resonant reactor, a capacitance of a resonant capacitor, a DC input voltage and a DC output voltage. Accordingly, many researches have been made on a relationship between these values and the resonant current. As the result of these researches, it has been found that in order to control the output voltage of the series resonant power converter at a constant voltage, without controlling the value of the output current, it is necessary to adjust the value of the output current (average value of the resonant current) by control of OFF-periods of a main switch means (that is, frequency control). However, in the case where the constant voltage control of the power converter is effected by the frequency control method, such problem arises that an operating frequency of the converter, which is in proportional relation to the output current, may drop to an audible range under a light load condition (in the case of less output current), with the result that a noise is produced.
In order to solve such problem, it has been proposed to provide a series resonant converter, as shown in FIG. 3, in which a tank circuit consisting of parallel resonant circuits having an infinite impedance at its resonant frequency is inserted in the series resonant circuit loop, whereby an impedance of the resonant circuit is increased under a light load condition and thus the load dependent characteristic of the operating frequency is minimized.
Now, the series resonant power converter according to a prior art will be explained, with reference to FIG. 3. The series resonant converter shown in FIG. 3 includes a first circuit 13 having switch means 11 and 12, such as bipolar transistors, connected together in series in forward direction, a second circuit 16 having diodes 14 and 15 connected together in series in forward direction, a third circuit 19 having resonant capacitors 17 and 18 connected together in series and a DC power source 21 connected across both ends of these circuits. The switch means 11 and 12 are arranged in forward direction relatively to the DC power source 21 but the diodes 14 and 15 are arranged to have opposite polarities. A juncture of the diodes 14 and 15 and a juncture of the resonant capacitors 17 and 18 are connected with each other, and a fourth circuit 24 is connected between the juncture 22 of the resonant capacitors 17 and 18 and a juncture 23 of the switch means 11 and 12. The fourth circuit 24 includes a rectifying circuit 25, a resonant inductor 26 and a tank circuit 27, which are connected in series with each other, and said tank circuit 27 consists of a parallel circuit including a resonant inductor 28 and a resonant capacitor 29. The rectifying circuit 25 consists of a bridge circuit including diodes 31-34. An output capacitor 35 is connected across output terminals of said bridge circuit and a load 36 is connected in parallel with said output capacitor 35.
Now, the operation of the above series resonant power converter will be explained.
As an initial condition, it is assumed that the resonant capacitor 17 has been charged up to a voltage of the DC power source 21 and the resonant capacitor 18 has been discharged to a zero voltage. Under such condition, if the semiconductor switch 11 is turned ON, a current i.sub.1 flows from the DC power source 21.fwdarw.the semiconductor switch 11.fwdarw.the diode 31 of the rectifying circuit 25.fwdarw.the load 36 (capacitor 35).fwdarw.the diode 33 of the rectifying circuit 25.fwdarw.the resonant inductor 26.fwdarw.the tank circuit 27 to the resonant capacitor 18. At the same time, a discharging current i.sub.2 flows from the switch means 11.fwdarw.the diode 31 of the rectifying circuit 25.fwdarw.the load 36 (capacitor 35).fwdarw.the diode 33 of the rectifying circuit 25.fwdarw.the resonant inductor 26.fwdarw.the tank circuit 27 to the resonant capacitor 17. This current is a resonant current which discharges the resonant capacitor 17 and charges the resonant capacitor 18. The capacitance C.sub.p of the capacitor 29 is set at a value higher than the capacitance C.sub.s of the resonant capacitor 17 (or the resonant capacitor 18), so that the voltage of the resonant capacitor 17 becomes zero and the voltage of the resonant capacitor 18 becomes equal to the power source voltage V.sub.i, after the lapse of about .sqroot.2L.sub.s .multidot.C.sub.s {.pi.-cos.sup.-1 (V.sub.o /(V.sub.i -V.sub.o))} sec, where V.sub.o is a voltage (output voltage) of the output capacitor 35, L.sub.s is an inductance of the resonant inductor 26 and V.sub.i is a voltage of the DC power source 21. At this moment, the diode 14 becomes conductive and the current which has passed through the resonant inductor 26, flows as the current i.sub.2 from the resonant inductor 26.fwdarw.the tank circuit 27.fwdarw.the diode 14.fwdarw.the switch means 11.fwdarw.the rectifying circuit 25.fwdarw.the load 36 (capacitor 35).fwdarw.the rectifying circuit 25. This current i.sub.2 is consumed by the load 36, until it becomes zero.
Thus a half cycle of the operation terminates. Then, if the switch means 12 is turned ON, the resonant capacitor 17 is charged while the resonant capacitor 18 is discharged and the similar operation as described above occurs. Thus the next half cycle terminates.
The resonant frequency f.sub.1 of the parallel resonant tank circuit 27 consisting of the capacitance C.sub.p of the capacitor 29 and the inductance L.sub.p of the inductor 28 is expressed by the following equation: ##EQU1## and the resonant frequency f.sub.0 of the series resonant circuit consisting of the capacitance C.sub.s of the resonant capacitor 17 (or the capacitor 18) and the inductance L.sub.s of the resonant inductor 26 is expressed by the following equation: ##EQU2##
With regard to the above resonant frequencies f.sub.1 and f.sub.0, it is required for them to satisfy the following relationship EQU f.sub.1 &lt;f.sub.0 ( 3)
To meet such requirement, f.sub.0 is set at a value substantially lower than f.sub.1 and the impedance of the tank circuit 27 is increased at around f.sub.1, so that the minimum operating frequency of the converter is clamped at f.sub.0. That is, the operating frequency of the converter should be controlled to be increased so that it becomes higher than an audible range, thereby preventing noise scattering.
On the other hand, to reduce the output current, the operating frequency of the converter should be controlled to be decreased. Under such circumstances, it has been a usual practice to generate parallel resonance in the tank circuit 27 so that the converter can operate under excessively light load and to restrict the input current of the converter to feed only less output current. By using such additional parallel resonant tank circuit, it is possible to operate the converter with good result even under light load condition, without the need of lowering the operating frequency of the converter to the audible frequency range. In general, in order to hold the output voltage or the output current at constant, it is usual to detect a variation thereof thereby automatically controlling the operating frequency.
The converter according to the prior art, as described above, poses some problems to be solved.
Firstly, in the circuit of the series resonant converter according to the prior art, in order to simultaneously satisfy the requirements as expressed by (1), (2) and (3), the capacitance of the resonant capacitor 29 in the parallel resonant tank circuit 27 must have a high value, at least four to five times as high as that of the resonant capacitor 17 or 18. Furthermore, the energy handled by the parallel resonant circuit 27 must have a value corresponding to Q times as high as the energy transmitted to the output side, where Q is the Q factor of the parallel resonant tank circuit. Accordingly, the resonant inductor used must be of large size, with the result that the power dissipation is increased. Furthermore, it is necessary to separately provide a resonant capacitor having a large capacitance, for the tank circuit 27.
Secondly, in the control method for controlling the series resonant power converter according to the prior art, in the case where a negative feedback is applied to the series resonant converter to control its output power, a temporary interruption of the wave form may happen at a transient time responsive to starting of operation, variation of the load condition, variation of input voltage or the like and the operating frequency may drop below the parallel resonant frequency of said tank circuit. In such cases, the impedance of the series resonant circuit loop drops when the operating frequency varies beyond the parallel resonant frequency of said tank circuit, so that the converter is operated in undesirable positive feedback operation rather than negative feedback operation and it becomes impossible to effect a stable control of the converter.